The safety critical elements are those parts of an installation or plants that can be subjected to failure and substantially cause, contribute, prevent or help recover from a major accident hazard or event. It is essential to develop performance standards for the identified SCEs. The SCE life cycle management involves alignment of maintenance, inspection and testing and performance history to maintain SCE in suitable working conditions. Continual monitoring helps in conformance of SCEs by Performance Standards.
The following steps as depicted in are used for the identification of SCEs:
This process allows the identification of all SCEs associated with the facility as well as detailing those systems deemed not safety critical with regard to causing or mitigating a MAE.
Note: There are several systems whose failed integrity could lead to serious harm/injury,
e.g. pressurised release, exposure to chemicals, etc, but not necessarily a MAE. These systems are not identified as an SCE, as the consequence due to failure of their integrity is not severe enough to lead or contribute ‘substantially’ to a MAE. This determination has been made by reference to the results of the HAZID, and engineering judgement and experience.
The recommended model for developing Safety Critical Element Performance Standards is based on the Functionality, Availability, Reliability, Survivability and Interaction (FARSI) format where:
Functionality refers to the purpose that the Safety Critical Element has to be able to perform to prevent, detect or mitigate a hazardous event or to protect people.
Availability refers to the proportion of the time that the Safety Critical Element will be required to perform on demand.
Reliability refers to how likely the Safety Critical Element is to perform on demand.
Survivability refers to how the Safety Critical Element will perform after a major accident has occurred, i.e. how well it will survive a fire, explosion, dropped object, etc.
Interaction refers to the way that the Safety Critical Element in question is dependent upon other SCEs to operate or otherwise interacts with other SCEs